This invention relates generally to the testing of components, and more particularly to methods and apparatus and for testing components having non-uniform surfaces.
Eddy current (EC) inspection devices are used to detect abnormal indications in a component under test such as, but not limited to, a gas turbine engine component. At least one known EC inspection device is used to detect cracks, pings, dings, raised material, and/or other surface imperfections on a surface of the component, and/or to evaluate material properties of the component including the conductivity, density, and/or degrees of heat treatment of the component.
During operation, known EC devices measure the interaction between an electromagnetic field generated by the EC device and the component being tested. For example, known EC devices include a probe coil that generates a magnetic field. When the coil is positioned adjacent to a conductive component, an eddy current is generated on the surface of the component. A flaw on and/or near the surface of the component generates a disruption in the eddy current field which produces a secondary field that is received by the eddy current probe coil or by a sensor coil in the eddy current probe which converts the altered secondary magnetic field to an electrical signal that may be recorded on a strip chart recorder for example.
At least one known EC device includes a relatively small coil that is typically 0.020 inches in diameter, that is used to detect surface flaws, surface contamination, material properties, and/or a surface roughness of the component being tested. In use, the coil is positioned normal to the surface of the component under test. A substantially constant pressure is applied to the probe as the coil moves along the surface of the component under test to facilitate maintaining an integrity of the signal generated by the EC device. However, when the EC device is not oriented normal to the surface of the component being tested, a “lift-off effect” may be created
To facilitate reducing the lift-off-effect, at least one known EC device includes a dual-coil probe, e.g. a differential probe, having a pair of coils with an opposite polarity. Each coil in the dual-coil probe generates an electrical signal when the probe contacts a surface of the component being tested. When the dual coil probe passes over a smooth surface of the component being tested, the signals cancel each other. However, when the dual coil probe passes over a local physical abnormality on the surface, the probe generates a signal that is proportional to the size, depth, etc., of the physical abnormality.
When a non-continuous component surface feature is inspected, such as a feature on a rotating part, known differential probes may have difficulty resolving sharp curvatures, in such areas as corners and cusps. During operation, when such probes encounter a corner or cusp, the differential probe device may become skewed to the surface of the component, such that a resulting lift-off effect may cause a loss of usable data. Accordingly, known EC devices may be less effective in generating an accurate response when the EC device is used to detect an abnormal condition on a component having complex geometries, and/or a component having irregular conditions, especially in components including sharp indexing or objects that extend into the path of the probe such that the probe cannot consistently be placed normal to scan surface.